This invention relates to food packaging materials in the form of flexible films bearing printed images.
In order to provide food packaging films with improved barrier properties, multi-layer films of two or more synthetic resins are normally used. Examples include coextruded polyethylene/polypropylene (“coex”) laminated to polyethylene terephalate (PET), PET laminated to LDPE (low density polyethylene), and metalized PET laminated to coex. Other such multi-layer food packaging films can be formed from nylon, metal foil, and ethylene vinyl acetate polymers and copolymers.
Where it is desired to provide such laminated packaging films with printed images, the image is normally printed on the outside of the film, i.e., on the major face of the film not facing the food in the package. When transparent or translucent packaging films are used, the image can be printed on the inside of the film instead.
To protect these printed images from damage due to rubbing, flexing, abrasion and heat sealing, these printed images are often covered with a suitable barrier coating or “trap-print film.” See, U.S. Pat. No. 7,063,882 to Mossbrook et al., the entire disclosure of which is incorporated herein by reference. If so, these trap-print films are normally made from materials suitable for direct food contact (“food compatible material”), even if the image is printed on the outside face of the packaging film. Because most packaging films are supplied in the form of large rolls, trap-print films on the outside face of a film contact the film's inside face when the film is wound up upon itself in forming the roll. Therefore, even if the packaging film is printed on its outside face, the trap-print film will normally be made from a food compatible material to prevent contamination of the inside, food contacting face of the same film.
Trap-print films can be applied to printed packaging films by any conventional technique. For example, trap-print films can be applied by extrusion coating, if desired. However, because of the time needed to set up the extruder and the considerable waste generated when the extruder is started and stopped, extrusion coating usually requires long production runs to be economically viable.
Trap-print films can also be applied by laminating with adhesive bonding. However, this approach limits the inks that can be used for printing to those which retain very low levels of solvent so as not to adversely affect the bond strength of the subsequently applied adhesive. These adhesives exhibit generally low bond strengths before cure, which may result in undesirable bubble formation and/or “tunneling.” In addition, full cure of these adhesives can be slow, which often requires these films to be stored before processing is complete, which in turn increases capital costs. In those situations where two component adhesives are used, these adhesives must be discarded if not applied quickly enough, which also increases costs.
In order to avoid these problems, it has also been proposed to form such trap-print films from electron beam activated coatings. However, election beam curing equipment is not widely available in the food packaging industry. Moreover, this equipment, as well as the materials used in electron beam coating in general, are expensive.